Abstract

Dr R. A. Chadwick writes: In a recent paper, Beach (1984) describes aspects of the structural evolution of the Witch Ground Graben. Figure 2 of his paper illustrates a structural section across the graben, the eseential features of which are reproduced here (Fig. la). Beach’s section is constrained in the shallower part by seismic reflection data, but beneath the base of Permian strata, the geometry of the basin-controlling faults has been deduced using balanced section criteria.

It is here suggested that, barring strike-slip movements of implausible geometry, the section is not viable. A family of late faults labelled L1–L7 on Fig. la, have southwesterly apparent dips, and cut and displace the Upper Cretaceous sequence. Beach shows them terminating, at depth, against a group of earlier faults (labelled E2–E4) which show northeasterly apparent dips. Beach suggests that the two groups of faults have a synthetic-antithetic relationship. This is unlikely. Since the faults labelled E do not displace the highest Cretaceous beds, any post-Cretaceous movement must have been restricted to the deeper segments of their fault-planes, below the intersections with the faults labelled L. This situation is mechanically implausible and is, moreover, refuted by Beach’s original section which shows an early fault (here labelled E3) to have the same throw both above and below the intersection with a late fault (here labelled L3).

Therefore, it is here concluded that the faults labelled E must be cut and displaced by the later faults labelled L. I have constructed an alternative section